FERTILITY 89 



average age at which the domestic animals cease to 

 breed is difficult to determine because of the scarcity of 

 records on this point. The persistence of the breeding 

 powers of the domestic animal is influenced by the condi- 

 tions under which they are reared. With favorable 

 conditions, the mare has produced young regularly until 

 twenty-five years old. Ewes have continued to produce 

 offspring until nineteen years of age. 



87. Confinement and fertility. The close confine- 

 ment of animals seriously interferes with their fertility. 

 Wild animals like the elephant, tiger, lion, squirrel and 

 monkey in captivity are often sterile. Darwin states 

 that the same animals breed more readily in traveling 

 shows than in zoological gardens. Among the domestic 

 animals, confinement and lack of exercise are frequent 

 causes of a low degree of fertility. It is generally a mis- 

 take to confine ewes or sows in a small lot during the 

 winter months, under conditions which make it impossible 

 for them to secure adequate exercise. The same may be 

 said of males, including stallions, bulls, boars and rams. 

 The comparative infertility of a stallion may often be 

 traced directly to the fact that he is kept stabled in a small 

 dark stall with no regular exercise. Mares both before 

 and after breeding should be given regular exercise. It 

 is better that they should work at some slow regular task 

 every day. 



In an investigation on the relation of exercise to the 

 development of the internal organs by Kulbs and Ber- 

 berrich, 1 it was found that in the case of dogs and swine 

 the size of the muscles and the weight of the heart and 

 liver were increased by exercise. It was also observed 

 that the color of the bone marrow was deepened. 



1 " Jahrbuch Wiss. und Prakt. Tierzucht," 6 (1911), p. 232. 



